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Quantifying Chaos to Understand Liquids

Thursday, October 12, 2017

For those readers in regions where autumn is quickly approaching, a pumpkin spice latte might be just the thing to help you relax. As scientists like Moupriya Das and Jason R. Green from the University of Massachusetts Boston know, however, zoom in on this seasonal treat and the world is anything but relaxing.“A still glass of liquid appears lifeless to the eye but is teeming with activity on the nanoscale,” says Green. “What we cannot see are molecules in ceaseless motion, chaotically wandering through the available volume.” The intermolecular forces that cause these seemingly random interactions on the nanoscale also determine the properties of liquids on our scale—like the stickiness of honey and the bubble-enabling surface tension of water—but it’s not always clear how that process works.

In research published last month in the American Physical Society’s journal Physical Review Letters, Das and Green demonstrate a new approach to studying liquids that is based on their chaotic nature, caused by these interactions. Their work could bring us closer to linking the chaos of the nanoscale to the properties of liquids as we experience them.

A snapshot showing a side view of one of the simulated systems.Image Credit: Moupriya Das and Jason R. Green.

Chaos has long been a part of studying gases, according to Green, but not liquids. Instead, theories of liquids are generally based on work by Johannes Diderik van der Waals, a Dutch physicist in the late 1800s and early 1900s. The van der Waals picture of a liquid says that the structure of liquid is determined by attractive and repulsive electric forces between the molecules. Molecules in a liquid have a weak, long-range attraction to one another, but this is overshadowed by a stronger, short-range repulsive force due to Coulombic forces and the Pauli exclusion principle.

In this recent work, Das and Green shed new light on this picture of a liquid. They modeled a variety of fluids in three dimensions and, using algorithms based on molecular dynamics, they simulated the motion of molecules in the liquids. From the results, they were able to quantify the amount of chaos in the motion of the particles. This was a big computational project and running the simulations took several months.

A snapshot showing five out of the sixteen sizes of systems the researchers simulated with molecular dynamics.Image Credit: Moupriya Das and Jason R. Green / Physical Review Letters.

In these simulations, liquid molecules were in endless motion, constantly interacting. As in the van der Waals picture, the molecules attracted one another from a distance, but then repelled one another when they got too close. The simulations revealed that repulsive forces are primarily responsible for not only the spatial arrangements of the molecules in a liquid but also most of the chaos in their motion.

“What our work shows is how measures of chaos in the dynamics of liquids not only reflect the van der Waals picture but give a way to quantify the relative importance of attractions and repulsions,” says Green. He continues, “The marriage of chaos and the van der Waals picture, we think, could open the door to new theories linking the macroscopic properties of liquids with the microscopic motion of their constituent particles.”

In other words, the scientists studied the structure of liquids from a completely different perspective, rooted in chaos, and ended up with results that aligned with the traditional van der Waals view. This suggests that the new perspective is valid and may be applicable to situations where the van der Waals picture breaks down, such as for complex fluids like gels and cell suspensions.

Complex fluids are an intriguing subset of matter called “soft matter” that can flow but doesn’t behave quite like you’d expect. A long-term goal of this new approach is to learn about the role intermolecular forces play in the formation of soft matter, and then use the information to design soft matter with specific properties. Existing soft materials are already used in a variety of applications like liquid crystals displays and packaging materials, but with tailored properties they could lead to exciting new possibilities.

So, the next time you unwind from a stressful day with a pumpkin spice latte, take a moment to remember that we can learn a lot from changing perspectives once in a while.